Method and apparatus for using pulse current to extend the functionality of a battery

ABSTRACT

A method to extend the functionality of a battery, the method comprising drawing power from the battery, and repetitively drawing a current pulse greater than the minimum conditioning current from the battery, thereby conditioning the battery.

FIELD OF THE INVENTION

[0001] The present invention relates to batteries, and morespecifically, to extending the functionality of a battery.

BACKGROUND

[0002] Batteries are used for many functions, to power portablecomputers, provide backup power, and power all types of portabledevices. However, batteries have a limited lifetime. After a period ofuse, most rechargeable batteries develop “voltage depression,” whichresults in the battery run-time decreasing after each recharge.

[0003]FIG. 1 illustrates a prior art voltage curve, for a new battery110 and an old battery 120. The turn-off voltage 130 is set, for examplefor a camcorder, at a level below the level of the fully chargedbattery. Thus, a new battery, as can be seen, takes an hour to reach theturn-off voltage 130. However, an old battery 120 drops down morerapidly, to reach the turn-off voltage 130 after a mere 2.5 minutes.Thus, the old battery cannot be used to power devices, since the useabletime is minimal.

[0004] The prior art to reduce the “memory” effect has been to deepdischarge the batteries, typically at a current discharge rate wellbelow the normal operating current level for a given application.Neither the battery run time or lifetime is enhanced by this. Inaddition, the standard practice of discharging a rechargeable batterydown only to about 1.12 volts, which is considered the fully dischargedlevel for new batteries, contributes directly to the battery “memory”phenomenon where older batteries have greatly reduced run time.

SUMMARY OF THE INVENTION

[0005] A method to extend the functionality of a battery, the methodcomprising drawing power from the battery, and repetitively drawing acurrent pulse greater than the minimum conditioning current from thebattery, thereby conditioning the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

[0007]FIG. 1 is a voltage diagram of a prior art battery before use andafter use.

[0008]FIG. 2 is a diagram showing the pulser coupled to a device.

[0009]FIG. 3A is a block diagram of one embodiment of the pulser.

[0010]FIG. 3B is a block diagram of an alternate embodiment of thepulser.

[0011]FIG. 4 is a more detailed block diagram of one embodiment of thecontrol circuit of the pulser.

[0012] FIGS. 5A-E are voltage and current diagrams of one embodiment ofthe response of the pulser.

[0013]FIG. 6 is an exemplary voltage and current diagram of an actualpulse response.

[0014]FIG. 7 is a diagram of the minimum conditioning current versusbattery voltage and load current, for a severely voltage depressedsystem.

[0015]FIG. 8 is a diagram of the minimum conditioning current versusbattery voltage and load current, for a new system.

DETAILED DESCRIPTION

[0016] A method and apparatus for extending battery functionality isdescribed. By defining a conditioning curve, which is a level of currentneeded to condition the battery, the system can successfully conditionbatteries to eliminate voltage depression. This extends thefunctionality of the battery significantly.

[0017] The battery functionality for a rechargeable battery includesbattery run-time, e.g. the time a single charge lasts, and useablebattery lifetime, e.g. the number of times the battery may be rechargedand have a useful discharge period. For the remainder of thisapplication, the term “conditioning” or “conditioner” will be used, andshould be understood to refer to extending the useable battery lifetimeand/or runtime.

[0018] Using the pulser, which will be described in more detail below,the battery has close to an ideal discharge cycle that eliminatesvoltage depression and enhances the lifetime and runtime of the battery.

[0019] The rejuvenation zone, the level of current needed to conditionthe battery and extend its functionality, modulates depending on thestate of the battery. The state of the battery includes the percentageof remaining charge in the battery, as well as the battery age and type.The minimum conditioning current needed for increasing the functionalityof the battery decreases as the voltage level decreases. If the currentbeing drawn from the battery is higher than this minimum conditioningcurrent, the battery functionality is extended. This is referred to inthis specification as “conditioning zone.”

[0020] For many batteries the nominal current is a close approximationof the optimum impedance matching point, where the battery voltage isreduced by 50%. For one embodiment, the power transfer is a function ofthe current density, which is cell size and battery type dependent.However, since current density is difficult to measure, using theimpedance matching point is an excellent, and easily measured,approximation. The actual minimum conditioning curve is battery agedependent. A typical curve for a voltage depressed battery is shown inFIG. 7, while a matching curve for a non-depressed battery is shown inFIG. 8.

[0021] In one embodiment, low duty-cycle high current load pulsesthroughout the discharge cycle pull current drawn from the battery abovethis minimum conditioning current level. These short duration pulsesprevent voltage depression from occurring, and increase thefunctionality of the battery. Furthermore, the pulses do not reduce theruntime of the battery, since they are low duty cycle. For oneembodiment, the duty cycle is typically less than 0.01%. The low dutycycle reduces the chances of overheating, or otherwise damaging thebattery.

[0022]FIG. 2 is a diagram showing the battery pulser coupled to adevice. The battery 210 is coupled to the device 230. The pulser 220 iscoupled between the battery 210 and the device 230. For one embodiment,the pulser 220 is only coupled between the battery 210 and the device230 for a short time, to condition the battery 210. After the batteryfunctionality has been increased—for one embodiment one full chargecycle—the pulser 220 may be removed, until the battery's voltagedepression again makes the use of the pulser 220 necessary. For anotherembodiment, the pulser 220 may be kept permanently between the battery210 and the device 230.

[0023] For one embodiment, the battery 210 may be any type of battery.For example, the battery may be a nickel based battery, such as nickelcadmium or nickel metal hydride. For another embodiment, the battery maybe a lead-acid battery. For yet another embodiment, the battery may be alithium ion battery.

[0024] In an exemplary application, the pulser 220 is contained in ahousing that is interposed between a Nickel Cadmium battery pack 210 anda device 230 such as a camcorder. The pulser 220 can be considered as anattachment to the battery pack 210 that maintains the battery 210 inoptimal condition.

[0025] The output of the pulser 220 may be disconnected by a switch,such that the current switching induced voltage spikes do not reach thedevice 230, for one embodiment. For another embodiment, for a devicethat is insensitive to voltage variations, this switch may beeliminated, and the voltage may be directly passed to the device 230.

[0026]FIG. 3A is a block diagram of one embodiment of the pulser. Thepulser shown is designed to be used with a device that is insensitive tovoltage variations. For example, this may be the case for power toolsthat have an electric motor as a load. This design may also be used forsystems in which the battery is not in use during the conditioningprocess. For example, the conditioning system may be implemented in abattery storage system, in which the battery is stored awaiting use.

[0027]FIG. 3B is a block diagram of one embodiment of the pulser. Thepulser 220 includes a control circuit 400 coupled between the positiveand negative poles of the battery. The control circuit 400, for oneembodiment, has two outputs 360, 390. The first output 360 controls afirst switch 350, while the second output 390 controls a second switch340. The first switch 350 couples the power from the battery to a deviceoutput 300 to which a device 230 may be coupled. Thus, when the firstoutput 360 is asserted, switch 350 connects the battery 210 and thedevice 230. When the first control circuit is deasserted, switch 350disconnects the battery 210 from the device 230. During this time,capacitor 330 powers the device 230.

[0028] The second switch 340 couples the a controlled current from theoutput of battery 210 to ground 340. Thus, when switch 340 is on, thebattery output goes to ground 345.

[0029] The control circuit 400 is designed to periodically draw a largecontrolled current from the battery 210. The level of the current isdesigned to be greater than the minimum conditioning current, which willbe described below. For one embodiment, the current drawn during thepulse is sufficiently large to reduce the voltage from battery 210 toone half the normally drawn voltage. Thus, if the battery 210 normallyprovides 6 volts, during the current pulse the voltage provided bybattery 210 is reduced to 3 volts.

[0030] For one embodiment, these current pulses have a short duty cycle,such that capacitor 330 can provide power to the output 300 during thetime when the battery 210 cannot provide a stable voltage. FIG. 5 belowillustrates in more detail the respective current and voltages, seen bythe battery 210, the control circuit 400, and the output 300, i.e. adevice coupled to the pulser 220.

[0031] For one embodiment, for circuits which are insensitive to voltagevariations, such as power tools, the battery disconnect switch 350 maybe eliminated.

[0032]FIG. 4 is a more detailed block diagram of one embodiment of thecontrol circuit 400. The control circuit 400 includes a voltageregulator 410, to set the FET switch 340 gate on-state voltage level,which determines the current pulse level. The voltage regulator 410 hasas an input the output of the battery. The output of linear voltageregulator 410 is coupled to driver 440. Driver 440, for one embodiment,uses complimentary P-channel and N-channel FETs (CMOS) in its output.When driver 440 is asserted, the internal P-channel FET is on, andconnects the output of the voltage regulator 445 to the gate of switch340. The P-channel FET has a sufficiently low ON resistance to insurethat there is virtually zero voltage drop across it, thus guaranteeingthat the linear regulator 410 output voltage 445 is accurately impressedon the gate of switch 340.

[0033] The battery output is coupled to inverter 450.

[0034] The control circuit 400 further includes a pulse generator 430.The pulse generator 430 is responsible for generating the current pulse,as well as a blocking pulse, as will be described below. Pulsegenerators 430 are known in the art.

[0035] The output of pulse generator 430 is input to a logical OR 460, alogical AND 470, and a delay 420. The logical OR 460 drives signal 360,while the logical AND 460 drives signal 390. As described above, signal360 controls switch one, while signal 390 controls switch two.

[0036] The output of delay 420 is the second input into logical OR 460and logical AND 470. The output of logical OR 460 drives inverter 450,while the output of local AND 470 drives driver 440. Thus, when both thedelay 420 and pulse generator 430 are on, signal 390 is asserted (one).When either the pulse generator 430 or the delay 420 is on, signal 360is asserted (zero). Thus, signal 360 starts earlier, by the delay, andends later by the delay, than signal 390. FIGS. 5A-E clarify thesesignal relationships.

[0037] As stated previously, the linear voltage regulator 410 in FIG. 4is used to set the current pulse level. The drain/source current ofswitch 340 is primarily determined by the transconductance of the FETand the gate to source voltage. By varying the driver voltage 445, thegate voltage of the FET is varied, and the drain/source current ofswitch 340 will vary in proportion to the gate voltage.

[0038] For one embodiment, for circuits which are insensitive to voltagevariations, such as power tools, the delay logic, the AND logic, the ORlogic, and the driver 450 may be eliminated. Then, the pulses generatedby pulse generator 430 may be directly coupled to the output, withoutisolating the device. In that case, the device sees the voltage spike atthe end of the current pulse, as well as the lowered voltage. If thedevice is not damaged by such variations in voltage, the circuit may besubstantially simplified.

[0039] FIGS. 5A-E are voltage and current diagrams of one embodiment ofthe response of the pulser. FIGS. 5A-E illustrate a single currentpulse, and the various responses to the current pulse. Typically, thepulse frequency is between 100 pulses per second and 1 pulse per minute.

[0040]FIG. 5B illustrates the battery current being drawn from thebattery during the pulse. Note that the pulse has a slew rate—the slopeof the pulse as it rises and falls—and is not perfectly rectangular. Theslew rate effects the overshoot 520 that is shown in the batteryvoltage, FIG. 5A.

[0041] For one embodiment, the pulse lasts approximately 25 μs. For oneembodiment, the pulse may be between 1 μs and 500 μs. Note that otherpulse widths may be used. Typically, pulse widths of over 500 μs causevoltage droop and internal heating in FET 340, which may raise thejunction temperature above the safe limit. Typically, pulse widths under5 μs require such a high slew rate that the overshoot 520 becomes toolarge. Thus, generally, the pulse rate is between 5 μs and 500 μs. Notethat the pulse width controls the amount of power transferred into thebattery. The amount of power needed for conditioning depends on thebattery type and the cell size.

[0042]FIG. 5A illustrates the battery voltage. As can be seen, thebattery voltage is significantly reduced during the current pulse. Forone embodiment, by decreasing the slew rate of the battery current 530,the voltage spike 520 is reduced.

[0043] The current switch drive 550, FIG. 5C, corresponds to signal 390,which indicates when battery current 530 starts to rise, and starts tofall. Current switch drive 390 pulls the current pulse from the battery.

[0044] The battery disconnect switch drive 560, FIG. 5D corresponds tosignal 360. As can be seen, the signal 560 starts prior to the currentswitch drive 550, and ends after the current switch drive 550. While thebattery disconnect switch drive 560 is active (e.g. low), the battery isdisconnected from a device coupled to the pulser. For one embodiment,the delay before and the delay after the current switch drive 550 isidentical, and determined by the delay set by delay unit 420.

[0045] Load device voltage 570, in FIG. 5E, is the voltage seen by adevice coupled to the pulser. As can be see, when battery disconnectswitch drive 560 is active, the battery is disconnected from the device,and the load device voltage 570 starts to slowly drop. The capacitor,resisting the change in voltage, maintains the voltage, and thus thevoltage level sinks slowly. When the battery disconnect switch drive 560is turned off, effectively reconnecting the battery and the device, thevoltage level increases to the previous level.

[0046] Note that the battery voltage spike 520 is not seen by the loaddevice voltage 570, because the battery disconnect switch drive 560disconnects the battery from the device, during the spike 520. For oneembodiment, there may be a very small spike.

[0047] For one embodiment, the capacitor is sized such that the voltagedroop 580 is minimized. For one embodiment, the voltage droop 580 isless than 1%. Thus, the device is not affected by the current pulse.

[0048] For one embodiment, for circuits which are insensitive to voltagevariations, such as power tools, the battery may remain coupled to thedevice, and the voltage levels seen by the device would be batteryvoltage 510, including spikes 520.

[0049]FIG. 6 is an exemplary voltage and current diagram of an actualpulse response. The voltage and current levels indicated are exemplary.As can be seen, the current increases for 4 μs to 20 Amperes. Thecurrent 650 may overshoot 660 slightly, which has no negative effect.

[0050] The battery voltage 610 decreases correspondingly, due to theinternal resistance of the battery. For one embodiment, the current 650is increased to drop battery voltage 610 to half its previous value. Bydecreasing the voltage 610 to half its previous value—3 volts from 6volts in this example—there is an impedance match between the batteryand the pulser.

[0051] The battery voltage 610 undershoots 620 slightly when the currentpulse is first started. For one embodiment, the undershot 620 is aresult primarily of internal battery capacitance, and other factors.

[0052] At the end of the pulse, there is a large battery voltageovershoot 630. The amplitude of the battery overshoot 630 may becontrolled by altering the slew rate at which the current turns off. Ifthe slew rate is decreased (e.g. slope 640 is made gentler, theamplitude of the voltage overshoot 630 may be decreased. This may beuseful for devices that are very sensitive to voltage variations. Note,however that for one embodiment the battery voltage 610, at the time ofthe overshoot 630, is isolated from any device coupled to the pulser, aswas described above.

[0053]FIG. 7 is a diagram of the minimum conditioning current curvesversus battery voltage and load current, for a highly voltage depressedbattery system. Note that this Figure does not illustrate the pulsesdescribed above. It shows the relationship of the minimum conditioningcurrents 730, 735 with respect to battery voltage 710. As can be seen,when the battery is fully charged, the minimum conditioning currents730, 735 are quite high, a large multiple of the standard battery loadcurrent 720. Therefore, the current pulses must be large, compared tothe standard load current.

[0054] The minimum DC current conditioning curve 730 illustrates theconditioning current needed, if a steady current were being pulled fromthe battery. The pulsed current conditioning curve 735 illustrates thecurrent levels needed for conditioning when the current is pulled fromthe battery using current pulses, as described above. As can be seen,the DC conditioning curve 730 requires higher currents than the pulsedconditioning curve 735. This is the result of the slew rate effect.Rapidly increasing and decreasing currents have a larger effect than asteady current. Thus, because the conditioning current is pulsed, ratherthan pulled as a steady DC current, a lower current level, and thereforeless power, needs to be pulled from the battery to effect conditioning.

[0055] Compare this to FIG. 8, illustrating a similar battery'srejuvenation curve, if the battery does not suffer from voltagedepression. As can be seen, the minimum DC current conditioning curve830 and minimum pulsed conditioning curve 835 retain their relationshipto each other. However, both are considerably higher in a new batterythan in a voltage depressed battery.

[0056] Thus, a prior art device that is unaware of the conditioningcurve, may accidentally hit the conditioning current level, for aseverely voltage depressed battery. However, as the battery becomesrejuvenated or conditioned, the conditioning curve moves up, requiringhigher and higher currents. Thus, without awareness of the batterycondition-dependence of the conditioning curves, prior art devices couldnot consistently condition batteries. They may ride the conditioningcurve, hitting it occasionally as the battery becomes more voltagedepressed. However, the battery is never fully rejuvenated using thismethod.

[0057] The system described conditions the battery, by using short,repeated, current pulses. For one embodiment, the current pulses areperiodic, e.g. every 10 seconds. For another embodiment, the currentpulses may occur at irregular intervals. However, repeated pulses areused, above the minimum conditioning current level. For one embodiment,the current pulses are designed to reduce the battery voltage, duringthe duration of the pulse, to half its normal value. This provides animpedance matching, which has been shown to be most effective for thebatteries tested. During the current pulse, while the battery voltage isreduced, for one embodiment, the battery voltage is disconnected fromany device coupled to the battery and pulser. A capacitor or similarsystem provides power for a device coupled to the battery during thistime. For one embodiment, the battery is disconnected a slight timeinterval before, and disconnected a small time interval after thecurrent pulse. This prevents the voltage spike that occurs at the end ofthe current pulse from affecting the device.

[0058] Note that although the above description states that the batterymay be in use during this conditioning process, it need not be in use.This conditioning may be done when the battery & pulser are not coupledto any device. In that case, for one embodiment, the simplified system,described above as FIG. 3A may be used, and the disconnection logic maybe eliminated.

[0059] For one embodiment, the battery may be conditioned 100%, at whichpoint the battery behaves like a new battery. However, in some cases,the battery may be partially damaged, or otherwise unable to be fullyconditioned. For example, if a battery has certain irreparable damage,some portion of the battery may not be conditionable. However, certaintypes of damage may be repaired using the above conditioning technique.For example, damaged cells may be restored, since the high currentpulses have the effect of reducing internal shorts, and in some caseseliminating them.

[0060] Thus, a typical conditioning may maintain the battery at 95%effectiveness, for example. For one embodiment, the typical conditioningresults vary by battery type, as well as the type of use that was madeof the battery. For example, a camcorder battery may have a lower levelof conditioning if the battery has been left discharged for an extendedtime, or if the battery has been stored in an excessively hot location,leading to battery damage.

[0061] Note that the voltages and current levels provided in FIGS. 5, 6,7, and 8 are exemplary, based on the expected results derived fromexperiments using 1000 mA/hour rechargeable NiCad batteries. It is to beunderstood that other batteries would have similar, but not identicalcurves.

[0062] In the foregoing specification, the invention has been describedwith reference to specific exemplary embodiments thereof. It will,however, be evident that various modifications and changes may be madethereto without departing from the broader spirit and scope of theinvention as set forth in the appended claims. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense.

What is claimed is:
 1. A method of conditioning a battery whiledischarging the battery, the method comprising: drawing power from thebattery; repetitively drawing a current pulse greater than the minimumconditioning current from the battery, thereby conditioning the battery.2. The method of claim 1, wherein the minimum conditioning current is afunction of the battery power remaining in the battery, and decreases asa function of a decrease of the battery voltage.
 3. The method of claim2, further comprising calculating the minimum conditioning current,comprising: drawing a first current level from the battery at a firstbattery power; determining whether a conditioning effect has beenobserved; and if the conditioning effect has been observed, determiningthat the first current level is at least at the minimum conditioningcurrent.
 4. The method of claim 1, wherein a pulse width is between 5 μsand 500 μs.
 5. The method of claim 1, wherein a pulse frequency isbetween 100 pulses per second and 1 pulse per minute.
 6. The method ofclaim 1, wherein the pulse occurs at irregular intervals.
 7. The methodof claim 1, wherein the battery is used to drive a device, while thecurrent pulses are being drawn.
 8. The method of claim 1, furthercomprising: isolating a device coupled to the battery from a voltage dipin response to the current pulse.
 9. The method of claim 8, wherein thedevice is powered from an alternative power source while the currentpulse is being drawn from the battery.
 10. The method of claim 9,wherein the alternative power source is a capacitor, and wherein thecapacitor is charged by the battery.
 11. A method of conditioning abattery to eliminate voltage depression, the method comprising: couplinga pulser between the battery and a device during use of the battery, thepulser repetitively drawing a current pulse greater than the minimumconditioning current from the battery, thereby conditioning the battery;and removing the pulser from between the battery and the device after aperiod of time, the battery having been conditioned during this processsuch that the in-use time of the battery corresponds to the in-use timeof the new battery.
 12. The method of claim 11, wherein the period oftime depends on an initial condition of the battery, such that less timeis needed to condition a fairly new battery compared to an old battery.13. The method of claim 12, wherein the period of time ranges from a fewminutes to multiple discharge cycles.
 14. The method of claim 11,wherein the battery is a rechargeable nickel cadmium (NiCad) battery.15. An apparatus to improve functionality of a battery comprising: apulser to repetitively draw a current pulse greater than a minimumconditioning current from the battery, thereby conditioning the battery.16. The apparatus of claim 15, wherein the minimum conditioning currentis function of the battery power, and decreases as a function of adecrease of the battery voltage.
 17. The apparatus of claim 16, whereinthe pulser comprises: a pulse generator to generate current pulses; andsuch that a battery voltage is reduced to half its normal value.
 18. Theapparatus of claim 17, further comprising: a switch to disconnect thepulser from a device coupled to the battery while the voltage isreduced; and a secondary power source to power the device during thepulse.
 19. The apparatus of claim 18, wherein the secondary power sourcecomprises a capacitor.
 20. The apparatus of claim 15, furthercomprising: a delay logic to disconnect the pulser from the device priorto the pulse, and to recouple the battery to the device after a voltagespike.
 21. An apparatus to condition a rechargeable battery during usecomprising: a pulse generator powered by the battery; a control circuitfor switching the pulse generator into the circuit periodically, topermit the pulse generator to repetitively draw a current pulse greaterthan the minimum conditioning current from the battery, therebyconditioning the battery.
 22. The apparatus of claim 21, wherein thebattery is a rechargeable nickel cadmium (NiCad) battery.
 23. Theapparatus of claim 21, further comprising: a second switch to decouplethe pulse generator an output of the apparatus, such that a devicecoupled to the output of the apparatus is not affected by a voltage dipas a result of the current pulse.
 24. The apparatus of claim 23, furthercomprising: a delay logic to decouple the pulse generator prior to thecurrent pulse, and to recouple the pulse generator after a delay, suchthat a voltage spike resulting from a decrease in the current after thecurrent pulse does not affect the device.
 25. The apparatus of claim 21,wherein the control circuit comprises: a first driver controlling thefirst switch, the first driver controlled by the pulse generator, thefirst driver disconnecting the pulse generator from an output of theapparatus during the pulse; and a second driver controlling a secondswitch to couple a capacitor to the output of the apparatus during thepulse, such that the capacitor provides power to a device coupled to theoutput of the apparatus.
 26. A method of conditioning a battery whiledischarging the battery, the method comprising: drawing a first currentlevel and a first voltage level from the battery; drawing a currentpulse at least ten times the first current level from the battery; suchthat the voltage provided by the battery during the current pulse is onehalf the first voltage level.